Hello Jean-Louis Gillet,
Thanks for your question.
Previews mean that that feature is not fully released and is typically available to users for testing and feedback before they become generally available.
Preview features are still in development, so they might be less stable, have bugs, or not perform as well as fully released features.
See a good 3rd party article I saw that explains the above on medium: https://vunvulear.medium.com/differences-between-ga-private-and-public-preview-on-azure-services-and-features-7ccdd42aa294
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola